Musnad Ahmad 26:383

Yahya ibn Ishaq narrated to us, Ibn Lahi'ah informed us, from Muhammad ibn Abdullah ibn Malik, from Sahl ibn Sa'd al-Ansari, from Abdullah ibn Mas'ud, that the Messenger of Allah (peace and blessings of Allah be upon him) used to say the taslim in his prayer to his right and to his left, until the whiteness of his cheeks could be seen.